Implantable hearing aids entail the subcutaneous positioning of some or all of various hearing augmentation componentry on or within a patient's skull, typically at locations proximate the mastoid process. Implantable hearing aids may be generally divided into two classes, semi-implantable and fully implantable. In a semi-implantable hearing aid, components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit a processed audio signal to implanted components such as a receiver and transducer. In a fully-implantable hearing aid, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, a processed audio signal is provided to a transducer to stimulate a component of the auditory system
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to mechanically stimulate the ossicles via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicles are made to mechanically vibrate, causing the vibration to stimulate the cochlea through its natural input, the so-called oval window. An example of this transducer is included in the MET™ hearing aid of Otologics, LLC, in which a small electromechanical transducer is used to vibrate the incus (the 2nd of the 3 bones forming the ossicles), and thence produce the perception of sound. In this case, the vibratory actuator is coupled to the ossicles during mounting and positioning of the transducer within the patient. In one example, such coupling may occur via a small aperture formed in the incus bone.
As will be appreciated, coupling with the ossicles poses numerous challenges. For instance, during positioning of the transducer, it is often difficult for an audiologist or surgeon to determine the extent of the coupling. In other words, how well the actuator is attached to the ossicles. Additionally, due to the size of the transducer relative to the ossicles, it is difficult to determine if loading exists between the ossicles and transducer. In this regard, precise control of the engagement between the actuator of the transducer and the ossicles is of critical importance as the axial vibrations can only be effectively communicated when an appropriate interface or load condition exists between the transducer and the ossicles. Overloading or biasing of the actuator can result in damage or degraded performance of the biological aspect (movement of the ossicles) as well as degraded performance of the mechanical aspect (movement of the vibratory member). Additionally, an underloaded transducer, e.g., where the actuator is not fully connected to the ossicles, may result in reduced performance of the transducer.
Another difficulty with such coupling is that in some cases patients can experience a “drop-off” in hearing function after implantation. Such a drop off may be caused by changes in the physical engagement of the actuator, e.g., due to things such as tissue growth, or may be caused by a malfunction of the transducer or other componentry. After implantation, however, it is difficult to readily assess the performance and/or adjust an implanted transducer and interconnected componentry. For example, in the event of a “drop-off” in hearing function after implantation, it is difficult to determine the cause, e.g., over/under loading of the interface due to tissue growth or some other problem with the hearing aid, without invasive and potentially unnecessary surgery. In addition, once coupled for an extended period, the maintenance and/or replacement with a next generation transducer may be difficult.